Planar optical arrangements giving rise to a synthetic image or an image that changes its appearance at different angles have been used in many applications. Besides purely esthetical uses, such arrangements have been used e.g. as security labels on bank-notes or other valuable documents, identification documents etc. The synthetic images, typically with a more or less pronounced three-dimensional character, have also been used for providing better geometrical understanding of complex shapes in e.g. two-dimensional information documents.
In the published international patent application WO 94/27254, a security device is disclosed. The security device comprises an array of microimages which, when viewed through a corresponding array of substantially spherical microlenses, generates a magnified image. This result is achieved according to the long known Moiré effect and was now applied to provide security labels with images having a three-dimensional appearance. The array may also be bonded to the array of microimages.
In the published US patent application US 2005/0180020, a micro-optic security and image presentation system is disclosed, being based on a similar basic idea. A film material utilizes a regular two-dimensional array of non-cylindrical lenses to enlarge micro-images or image data bearer structures of an image plane. By adapting the focal properties of the lenses, the distance between the lenses and the image plane, the diameter of the lenses, different magnifications, field of view etc. may be changed. Optoparallactic motion is achieved by having discrepancies in aligning between image plane and lens plane.
Images of this kind are also used as sheet material for e.g. packages. The packages achieve in such a way an eye-catching appearance, which typically is desired when marketing fancy or expensive products. If the synthetic images are made three-dimensional, the images may e.g. be configured to appear to be suspended inside (or even outside) the packages. However, since the arrangement utilize optical effects achieved with a planar configuration of image data bearer structures and lenses, such images have so far been limited to packages having planar surfaces. The synthetic image will typically deteriorate when applied e.g. on a curved bottle surface.
When the optical arrangement is used as a security device, it is important that the images that are perceived behave in a manner that is difficult to copy with simple means, but which are easy for the user to observe. Few security devices in prior art fulfill such requirements, if any at all. There are needs for security devices with unique structures and properties that are easy to detect and verify, but difficult to tamper or copy.
In the published European patent application EP 0 216 626, an image sheet for tamper-evident packages is disclosed. The sheet comprises a pattern of glass microspheres provided on top of a film, which microspheres function as microlenses. Axial markings are caused to be created at a sheet in contact with the rear of the microsphere by exposing the sheet for high-energy radiation. By making such axial marking creation when the film is held in a curved shape, an integral image, composed by the axial markings viewed through the microspheres, will be destroyed if the curvature is changed. If such a film e.g. is provided over the opening of a container, the existence of an image ensures that the container content is not tempered. A disadvantage of such an arrangement is that the image creation has to be performed when the film is applied at its final substrate. This makes efficient mass production very difficult. The approach of creating the axial markings by irradiation through the microspheres also puts severe limitations of what kind of images can be created. Furthermore, since the creation of axial markings has to be performed by intense irradiation at the final product, this kind of security device cannot be used for irradiation sensitive goods.